Not sure, but have you tried: \definefallbackfamily [mainface] [serif] [Gentium Plus] [range=greekandcoptic]
The wiki has a page on fallback fonts: https://wiki.contextgarden.net/Command/definefontfallback Searching for "greek" has numerous hits, including greekextended. Have you verified that the Greek glyphs are in the fallback font you've selected? Sometimes I'll use a site like FontDrop! to make sure that there are, in fact, glyphs in the character ranges specified. https://fontdrop.info/ Good luck!
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